As seen in FIG. 1, one or more dies are formed in a conventional manner on a wafer which, in turn, is formed from a semiconductor material such as silicon. Each die has an integrated circuit or device that has been formed but not yet detached from the wafer. Further, each die on the wafer can be tested by placing a set of mechanical probes in physical contact with the die's bond pads. The bond pads provide a connection point for testing the integrated circuitry formed on the die. The probes apply voltages to the input bond pads and measure the resulting output electrical signals on the output bond pads. Not all bond pads on a die, however, are easily accessible by these devices. Given the arrangement of the dice in FIG. 1, for example, it is generally easier to probe the long side of each die, as the short side of each die is usually too close to the other dice to allow sufficient clearance for testing purposes. Thus, it can be difficult to test circuits that are coupled to an inaccessible bond pad.
Requiring bond pads to be located only in the areas accessible during testing may lead to inefficient and complex circuit layouts. One known solution, as shown in FIG. 3, is to attach another bond pad, one that can be reached by a testing device, to the same wire used by the original bond pad. This solution, however, tends to increase the input capacitance. Attempts at minimizing this capacitance will result in the use of more die space.
A second known solution is to multiplex (mux) two input buffers together, as illustrated in FIG. 4, once again allowing a testable bond pad to access circuitry. With this mux circuit, however, signals from the original bond pad take longer to reach the die's integrated circuitry. In addition, if input is designed to be received from multiple input buffers arranged in a parallel configuration, this muxing solution would require duplicating large portions of the input circuitry, once again taking up a great deal of die space.